pkgdev commit: support iterating across target restrictions, generating commits per scope level

[radhermit]

Probably makes sense to limit it to package scope on the initial implementation. As a rough overview, the workflow would be similar to this:

1. Lots of ebuild changes are made across a repo, perhaps with some script or tool. For whatever reason, commits related to the changes are wanted per package instead of combining all the changes into a single commit.
2. pkgdev commit 'media-gfx/*' iterates over all media-gfx pkgs creating commits per package if changes exist (-u or -a would probably be implied).
3. All other changed ebuilds in the repo would be ignored.

Similar to pkgcheck scan, it should be simple to support path-based or extended-atom restrictions for target args.